1. Field of the Invention
This invention relates generally to expansion joints for piping systems. More specifically this invention relates to expansion joints having multiple sealing elements that allow relative movement between ends of the expansion joint.
2. Description of the Prior Art
Piping systems routinely expand and contract in response to changes in temperature, internal or external loadings and anchor movements. Many piping systems incorporate expansion joints that accommodate the expansion and contraction of the piping system by permitting relative movement between segments of the piping system. Common types of expansion joints include bellow type expansion joints and packed type expansion joints.
Packed type expansion joints consist of two overlapping pipe sections and packing material that fills the gap between the two pipe sections and provides a seal between the internal and external environments of the piping system. In most packing arrangements a flared section at the end of the overlapping pipe receives the packing material and a ring or collar urges the packing into the flared opening to provide the seal. At times, a packed joint can provide a good seal with only a relatively small compressive force. However, due to irregularities in the pipe, packing and compressive force application, a substantial compressive force is needed to provide a good seal. Thus, the ring ordinarily places relatively high compressive loads on the packing material. These high compressive loads create a high resistance to relative movement between the overlapping pipe sections. This high resistance, usually referred to as break-away force, can impose large forces on the piping system as it expands and contracts.
Another type of expansion joint uses a flexible sealing element, usually in the form of a bellows, to permit relative movement between pipe segments. When displaced, a bellow acts somewhat like a low k-factor (spring rate) spring and, together with any pressure loading from the piping, imposes a relatively constant force on the attached piping elements. The constant force associated with the bellows type expansion joint is usually much less than the break away force of a packed type expansion joint.
A bellow type expansion joint can be arranged such that the piping system pressure acts on the inside or outside of the bellows. U.S. Pat. No. 4,046,407 depicts a system where the piping system pressure acts on the outside of the bellows.
In order to function the bellow must have a relatively thin wall in relation to the piping system. Thin bellows are susceptible to stress corrosion or chemical attack. This thin wall makes the bellows a weak link in the piping system that can rapidly and catastrophically fail. Apart from rendering the piping system inoperative, catastrophic bellows failures can impose serious safety hazards. Bellows failures in high temperature piping systems or confined piping areas, such as steam pipe tunnels, are especially dangerous.
As a safeguard against bellows failure, some expansion joints provide double bellows elements that operate in parallel. In such arrangements the expansion joint employs two independent bellows usually having the same ply thickness which is sized to withstand the total pressure loading of the system. Such an arrangement is shown in U.S. Pat. No. 4,511,162 issued to Broyles. In the Broyles patent, two independent bellows straddle a space between the pipe ends of the expansion joint. Broyles also provides a pressure tap to monitor the pressure in the gap between the bellows and thereby sense if a leak has developed in the bellows.
Although the double element expansion joint offers some protection against catastrophic bellows failure, the protection in many cases may not be substantial. Metal fatigue causes many bellows failures. A parallel arrangement for a double bellows expansion joint exposes both of the expansion joint elements to the same number and type of fatigue cycles. Therefore, both bellows elements approach fatigue failure at about the same time. If both bellows elements do not fail at the same time a catastrophic failure is happily avoided. Nevertheless, the failure of one bellows signals imminent failure of the other bellows and prudence dictates a shutdown of the system for replacement of both expansion joint bellows. Consequently, the failure of a single bellows ply in a double bellows expansion joint can still burden the piping system operator with an inopportune system shutdown. Moreover, many bellows failures occur at the point where the bellows attach to the pipe elements -- a double element expansion joint provides no protection against failure of an expansion joint at the bellows to pipe attachment point which is generally the point of highest stress.
Therefore, expansion joint elements have posed the problem of high displacement forces, in the case of packed joints or, in the case of bellows type expansion joints, the hazard of catastrophic failure or the inconvenience of unplanned shutdowns.